1. Field of the Invention
This invention relates to electrorheological fluids, i.e. fluids which exhibit a significant change in flow properties when exposed to an electric field. These fluids are also known as "electric field responsive fluids," "electro-viscous fluids" or "jammy fluids."
2. Background
Early studies of electrorheological fluids (ERFs) were performed by W. M. Winslow who demonstrated that certain suspension of solids (the "discrete," "dispersed" or "discontinuous" phase) in liquids (the "continuous" phase) show large, reversible electrorheological effects. These effects are generally as follows: in the absence of an electric field, electrorheological fluids exhibit Newtonian flow properties; specifically, the shear stress (applied force per unit area) is directly proportional to the shear rate applied (relative velocity per unit thickness). When an electric field is applied, a yield stress phenomenon appears and no shearing takes place until the shear stress exceeds a minimum yield value which increases with increasing field strength, i.e. the fluid appears to behave like a Bingham plastic. This phenomenon appears as an increase in apparent viscosity of several, and indeed many, orders of magnitude.
Electrorheological fluids change their characteristics very rapidly when electric fields are applied or released, typical response times being on the order of 1 millisecond. The ability of electrorheological fluids to respond rapidly to electric signals make them uniquely suited for use as elements in electro-mechanical devices. Often, the frequency range of a mechanical device can be greatly expanded by using an electrorheological fluid element rather than an electro-mechanical element having a response time which is limited by the inertia of moving mechanical parts. Therefore, electrorheological fluids offer important advantages in a variety of mechanical systems, particularly in those which require a rapid response between electronic controls and mechanical devices.
A range of devices have been proposed to take advantage of the electrorheological effect. Because of the potential for providing a rapid response interface between electronic controls and mechanical devices, it has been suggested that these fluids be applied in a variety of mechanical systems such as electro-mechanical clutches, fluid-filled engine mounts, high speed valves with no moving parts, and active dampers for vibration control, among others.
A wide range of combinations of liquids and suspended solids have demonstrated electrorheological effects. The basic ingredients of prior art electrorheological fluids are fine dielectric particles, the surfaces of which typically contain adsorbed water or some other surfactant or both, suspended in a non-polar dielectric fluid having a permittivity of less than that of the particle and a high breakdown strength. As used herein, the term "dielectric" refers to substances having very low electrical conductivity. Such substances have conductivities of less than 1.times.10.sup.-6 mho per centimeter. These are general system requirements and accordingly a variety of systems have been found to demonstrate electrorheological effects.
While a number of theories have been proposed to explain the electrorheological effect, a comprehensive theory explaining all of the observed phenomenon has not yet been developed. However, those of ordinary skill in the art are aware that certain system parameters affect the electrorheological response of any given electrorheological fluid. These parameters include, amongst others, the size and concentration of the particles (or discrete phase), the polarizability of the particles, the aspect ratio of the particles in the electric field, the particle surface area, the particle solubility or dispersibility in the continuous phase, the particle porosity and adsorbed moisture, presence of surface activators and surfactants, the rate of shear, the electrorheological fluid temperature and the strength of the applied electric field.
While it is known that the continuous phase should be hydrophobic, experimental evidence suggests that the electrorheological effect is related to water adsorbed to the solid particles or discrete phase. Consequently, early and, indeed, many currently proposed electrorheological fluids include adsorbed water in the discrete phase. For example, U.S. Pat. No. 4,483,788 to Stangroom et al relates to electrorheological fluids comprised of a water-containing polymer such as phenol-formaldehyde polymer as the discrete phase and an oleaginous hydrophobic fluid as the continuous phase. It is specified that a discrete phase content of 25-35% by volume is preferred. However, the electrorheological effect of these fluids using polymers as the discrete phase is limited by the extent of polarizability of the polymeric molecules, the aspect ratio of the polymer in the electric field, the particle size of the polymer, its surface area and the dispersibility of the polymer in the continuous phase.
The scope for practical application of adsorbed water-dependent electrorheological fluids is, however, limited since many devices in which such fluids may be of use are more desirably operated at relatively high operating temperatures and relatively high electric field strengths.
Some efforts have been directed toward developing electrorheological fluids which do not rely upon the presence of adsorbed or free water and which require relatively low electric field strengths. For example, U.S. Pat. No. 4,722,407 to Carlson discloses an electrorheological fluid which includes (1) a dispersed particulate phase of a polarizable solid material which conducts current along only one of its three axes; and (2) a continuous phase of a dielectric liquid. The Carlson electrorheological fluid operates in the absence of free water and is therefore suitable for use at temperatures at which water-containing electrorheological fluids cannot operate because of the evaporation of the water. The electrorheological fluid is also said to require a relatively low electric field strength. The preferred polarizable solid material is lithium hydrozinium sulfate. However, a "stabilizer" is necessary to suspend the lithium hydrozinium sulfate which otherwise tends to settle out in the continuous phase. In the absence of this stabilizer, the lithium hydrozinium sulfate-based electrorheological fluid forms a heavy, flocculated grease. When too much stabilizer is added, the mixture separates into two layers: a sediment layer containing the lithium hydrozinium sulfate and a clear layer. While it is not clear from the Carlson patent, it is known that sulfactants generally increase fluid conductivity resulting in resistive heating of the solution which reduces the electrorheological effect.
U.S. Pat. No. 4,744,914 to Filisko et al discloses an electrorheological fluid which may be used at temperatures in excess of 100.degree. C. (typically about 120.degree. C.) which includes (1) a non-conductive liquid phase; and (2) a crytalline zeolite particulate phase. The preferred zeolite is of the formula: EQU M.sub.x/n [(AlO.sub.2).sub.x (SiO.sub.2).sub.y ].multidot.wH.sub.2 O
where M is a metal cation or mixture of cations of valence n; x and y are integers and y/x is from about 1 to about 5; and w is a variable. The zeolites of Filisko are dried under vacuum at between about 250.degree. to 350.degree. C. and are not entirely water-free but the residual water does not evolve at operating temperatures. The examples indicate the use of the zeolite in concentrations of 10 to 16 g per 20 ml of liquid phase.
While Filisko does not mention the problem of phase separation often associated with the use of particulates as a discrete phase, it is noted in U.S. Pat. No. 3,412,031 to Martinek et al. The Martinek ERF includes a discrete phase of surface-modified silica gel and discloses that the addition of small amounts of a carboxylic acid has a "slight beneficial effect upon the phase stability of the product formulation." ERF's compounded with carboxylic acids can be stored for long periods of time without separation of the silica. The Martinek invention also requires the addition of a nitrogen-containing organic compound when the ERF is to be activated by a constant potential. Among the listed compounds are the primary, secondary and tertiary amines, aminoethers, aminoalcohols and diamines.
U.S. Pat. No. 4,668,417 to Goossens et al. is directed to an ERF containing silica gel and a polymer as a dispersing agent which prevents or minimizes the settling of the silica and enables ready redispersion of the silica in the event of settling. The disclosed polymers are soluble in liquid hydrocarbons (i.e. the continuous phase) and contain 0.1 to 10% N and/or OH groups, from 25-83% C.sub.4 -C.sub.24 alkyl groups and have molecular weights in the range of 5.times.10.sup.3 to 10.sup.6. Significantly, however, the ERF yet requires the use of about 40 wt. % fine silica, which is an inherently abrasive particle, so that the Goossens ERF may be expected to be abrasive in use.
There yet exists a need for an electrorheological fluid that will operate at the relatively high temperatures encountered in commercial applications that is stable in the sense that the discrete phase will not settle out of the ERF composition, that is non-corrosive and non-abrasive in use, while requiring a low electric field strength to produce a relatively high change in viscosity and wherein the discrete phase is present in low concentration.